Yield execution in mutation_result_merger
Yield execution in mutation_result_merger

mutation_result_merger::get can run for a long time. Make it yield
execution from time to time.

Message-Id: <a57ca01d7249f3e90639ef7f299a24a125dfaefb@scylladb.com>

kjniemi/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla